I. Field of the Invention
The present invention relates to RF amplifiers. More particularly, the present invention relates to a novel and improved method and apparatus for RF amplification of a wireless phone transmit signal.
II. Description of the Related Art
A mobile unit or wireless phone integrates numerous complex circuits. An RF transceiver is used to provide the wireless communication link with base stations. The RF transceiver is comprised of a receiver and a transmitter. The antenna, interfaced to the mobile unit, receives the RF transmission from the base station and outputs the signal to the receiver. The receiver amplifies, filters, and downconverts the received signal to baseband. The baseband signal is then routed to a baseband processing circuit. The baseband processing circuit demodulates the signal and conditions it for broadcast through a speaker to the user.
User input via keypad presses or voice input to a microphone is conditioned in the baseband processing circuit. The signal is modulated and routed to the transmitter. The transmitter takes baseband signals generated in the mobile unit and upconverts, filters, and amplifies the signal. The upconverted and amplified RF signal is transmitted to the base station through the same antenna as used for the receiver.
The design requirements of sustained high quality voice and data transmission must be balanced against the design requirements of battery operation, small size, low cost, and high reliability. The requirement that a mobile unit operate under battery power presents various issues that must be addressed. Batteries can only provide a limited amount of energy. Minimizing power consumption is the only way to extend battery life.
Increasing consumer demands to reduce the size and weight of a portable phone put further pressure on the hardware designer to reduce the size and weight available for a battery. Advances in battery technology allow for some reclamation of available energy storage lost due to size reduction. However, the overall trend is towards decreased phone power consumption as a means for extending phone talk and standby times while simultaneously reducing the form factor of the phone.
The transmit RF amplifier chain is one of the greatest sources of power consumption in a mobile phone. The design of the transmit RF amplifier chain in a Code Division Multiple Access (CDMA) mobile phone is further constrained by specifications imposed on output power range and output emissions. Specifications constraining mobile phone design include Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) IS-95-B MOBILE STATION-BASE STATION COMPATABILITY STANDARD FOR DUAL-MODE SPREAD SPECTRUM SYSTEMS as well as TIA/EIA IS-98-B, RECOMMENDED MINIMUM PERFORMANCE STANDARDS FOR DUAL-MODE SPREAD SPECTRUM CELLULAR MOBILE STATIONS. However, comparable specifications exist for mobile phones operating in other than the cellular frequency band as well as for mobile phones utilizing alternative modulation schemes such as Time Division Multiple Access (TDMA) or analog Frequency Modulation (FM). The specification covering the operation of a CDMA system in the Personal Communication Systems (PCS) band is the American National Standards Institute (ANSI) J-STD-008 PERSONAL STATION-BASE STATION COMPATIBILITY REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL COMMUNICATIONS SYSTEMS. Similarly, the phone, or personal station, is specified in ANSI J-STD-018, RECOMMENDED MINIMUM PERFORMANCE REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL STATIONS.
A successful amplifier design must satisfy all required specifications as well as minimize power consumption. One method of minimizing power consumption within an RF power amplifier is to utilize an efficient amplifier topology. Class A power amplifiers provide the best linearity but also are the most inefficient. Class AB power amplifiers provide increased efficiency over class A amplifiers at a cost of increased output distortion. The non-linear characteristics of the class AB power amplifier make it suitable only as the final high power amplifier in the transmit RF amplifier chain. Other amplifier topologies are not suitable for a CDMA mobile phone operating within the TIA/EIA IS-95-B specification because they are unable to meet the linearity requirements.
However, one single amplifier is incapable of providing the gain, output power, and linearity required in the transmit path. Therefore, several amplifiers must be cascaded in series to satisfy the transmit path requirements. What is required is a cascaded RF amplifier configuration that maintains the linearity necessary for CDMA communications while minimizing power consumption in order to maximize battery life.